The following question is taken from the June 2013 exam paper. An additive time series has the following trend and seasonal variations: Trend Y=4,000 + 6X where Y= sales in units X is the number of quarters, with the first quarter of 2014 being 1, the second quarter of 2014 being 2 etc. Seasonal variation Quarter 1 2 3 4 Quarterly variation (units) -4 -2 +1 +5 What is the forecast sales volume for the fourth quarter of 2015?
A、4,029
B、4,043
C、4,048
D、4,053
A、4,029
B、4,043
C、4,048
D、4,053
【参考答案及解析】
The correct answer is D. This is calculated by firstly computing the trend for fourth quarter of 2015 (Y = 4,000 + 6 x 8 = 4,048) and then adding a seasonal adjustment of 5, to give forecast sales of 4,053. The most popular choice was alternative C. This indicates that many candidates were able to calculate the trend but failed to apply the seasonal adjustment. Distracters (the incorrect answers to objective test questions) are often based upon partially complete calculations. Candidates are advised not to stop thinking as soon as they generate a number that corresponds with one of the options offered. A good way of avoiding this trap is not to look at the answers until you are satisfied that you have fully completed your calculation. Answer A used a value of 4 for period 4 of 2015 and generated the wrong trend figure, but then correctly processed the seasonal adjustment. Finally a minority of candidates selected answer B, indicating that they could correctly calculate trend but subtracted rather than added the seasonal adjustment.
The correct answer is D. This is calculated by firstly computing the trend for fourth quarter of 2015 (Y = 4,000 + 6 x 8 = 4,048) and then adding a seasonal adjustment of 5, to give forecast sales of 4,053. The most popular choice was alternative C. This indicates that many candidates were able to calculate the trend but failed to apply the seasonal adjustment. Distracters (the incorrect answers to objective test questions) are often based upon partially complete calculations. Candidates are advised not to stop thinking as soon as they generate a number that corresponds with one of the options offered. A good way of avoiding this trap is not to look at the answers until you are satisfied that you have fully completed your calculation. Answer A used a value of 4 for period 4 of 2015 and generated the wrong trend figure, but then correctly processed the seasonal adjustment. Finally a minority of candidates selected answer B, indicating that they could correctly calculate trend but subtracted rather than added the seasonal adjustment.